BishopBerkeley: According to http://www.bookup.com/pro_deo.htm :
"The Dutch maker of the famous Rebel line of chess engines, Ed Schröder, has retired after 20 years(!) of chess programming and has released a free version of his latest engine which he calls Pro Deo."
Pro Deo is generally regarded as one of the strongest free Chess engines available. In addition, it is a UCI engine, which means it can be run in the Chessbase interfaces ( Fritz (Computer) , Junior (Computer) , Shredder (Computer) , etc), the Chessmaster (Computer) interface, or the Winboard environment.
Just as Pro Deo is a later incarnation of REBEL (Computer) , so Rebel is widely believed to be a later incarnation of Mephisto (Computer) , one of the first (if not THE first) computer Chess programs to perform impressively against Grandmasters.
The official Pro Deo download site is:
though at the time of this writing, the site is down. (Good to check back later.)
To see how Pro Deo is doing in competition, you might check out these two ratings lists, which are frequently updated:
All rather fun!
(: Bishop Berkeley :)